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for this animation I used a smoke sim and a volume material. The material only selects part of the density values using a color-ramp node. The smoke has no temperature difference but is moved by a turbulence force-field

For this animation I used the gooseberry-branch of blender 2.75. I made a volumetric material using the point-density texture node to create a cloud and added some red point lamps to a second particle sytem from the same emitter.